This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / A Canterlot Wedding Aftermath
The immense breakdown Twilight suffers throughout the story. It's pretty tragic seeing her become so psychologically broken due to the events of the wedding, but what makes it worse is the fact on that it was caused by her own best friends, her brother, and her own mentor.
Twilight's friends saying their goodbyes when she is given the choice to stay in Ponyville or start over in Canterlot.
Twilight crying while talking to the therapist is made worse when her own friends and brother overhear this and start to cry knowing they did this to her..